This invention relates to a composition and method for removal of acid-corrosive substances from boiler flue gases and like vapors which are flowing in contact with materials that are subject to attack.
A primary example of the damage prevented by the present invention is the corrosion of the "fireside" metal surfaces of heat recovery equipment due to the presence of sulfur trioxide in the boiler flue gas. The sulfur trioxide which results from the combustion of most fuels combines with the moisture vapor in the combusted gas to produce sulfuric acid. This condenses onto the various surfaces at the "cold end" of the boiler when the flue gas temperature falls below the condensation temperature or "dew point" of the acid vapor. Resultant "fireside" corrosion is a principal limiting factor in the use of heat recovery equipment.
Thermal energy recovery equipment is commonly used to maximize the recapture of combustion heat, as the greatest loss of efficiency in boilers is due to heat energy escaping in the exiting gas. To operate a boiler at maximum efficiency, it is necessary to reduce this heat loss to an absolute minimum, a one percent fuel saving being realized for each 40.degree. F. reduction in flue gas temperature.
In order to conserve as much combustion gas heat as possible, it is customary to install, as heat recovery equipment in a boiler or generating unit, two separate recovery systems. First, an economizer is located in the gas passage between the boiler and the stack and is designed to recover a portion of the waste heat from the products of combustion and transfer this heat through a series of metal tubes to the inlet water which flows to the boiler. Second, an air preheater is located downstream from the economizer and consists of metal plates or tubes having hot exit gases on one side and incoming air on the other. The air preheater increases the temperature of the air feeding the combustion system and increases the overall efficiency of the boiler.
Dilute sulfuric acid is known to possess a "dew point" about 200.degree. F. higher than that of water. This "dew point" varies with the relative proportions of the acid and the water vapor. Measured dew points range from 100.degree. to 400.degree. F., the higher values occurring with higher sulfur content fuels. The potential rate of corrosion is known to increase with an increase in the dew point. A minor amount of sulfuric acid conversion takes place at elevated temperatures (to 620.degree. F.). The measured difficulties occur, however, at temperatures below the acid "dew point" (which may vary from 280.degree. to 320 F.) under normal operating conditions.
Generally, corrosion is most noticeable at or after the air heater, for this is the point in the exit gas flow where the metal temperatures fall below the dew point of the acid vapors.
A number of means for minimizing the rate of boiler corrosion have been developed in the past. Since corrosion occurs on the lowest temperature surface, air heater designs have been developed which incorporate replaceable cold end sections. Other means for minimizing corrosion are aimed at increasing the metal temperature. However, this, of course, tends to defeat the economy of the air preheater system.
To avoid the necessity of using corrosion-resistant or replaceable metal parts in the cold end of the boiler, various additives have been suggested to reduce the acid content of flue gases. British Pat. No. 822,314 mentions the prior use of ammonia or tar bases injected into the exit flue gases and notes the disadvantage of the objectionable smell of these compounds. As a solution to these problems, the British Patent suggests the use of alkali metal compounds such as sodium or potassium hydroxide. These caustic agents, while perhaps reducing the acid content of flue gases, provide handling problems and may, if used in excess, contribute to corrosion themselves.
According to U.S. Pat. No. 2,992,884, and alkalized alumina absorber can be used to convert sulfur dioxide to sulfur trioxide and then to aluminum sulfate in the flue gas of a boiler. The absorber is subsequently reacted with a reducer gas and regenerated, but at the cost of an additional plant facility.
U.S. Pat. No. 3,837,820 discloses the burning of fuel in the presence of compounds of manganese and magnesium, with additional amounts of these compounds being added to the flue gases. Due to the cost of these metals and their compounds, the expense of this system tends to negate the advantages of corrosion reduction.
According to U.S. Pat. No. 3,886,261, the sulfur trioxide in flue gas is treated with alumina (Al.sub.2 O.sub.3); and this method has resulted in some corrosion protection as shown by probes inserted at the cold end of the boiler. However, this patent suggests that the protection afforded results from the deposition of alumina particles on the surfaces of the metallic structures which comprise the cold end of the boiler, rather than a reduction in the level of sulfur trioxide in the flue gas.
U.S. Pat. Nos. 4,140,750; 4,134,728; and 4,134,727 present, respectively, the addition of sodium metasilicate; n-aminoethyl ethanolamine; and mixtures thereof to flue gases to reduce the content of sulfur trioxide. According to these inventions, solutions of the aforementioned substances are dispersed in the flue gases through the use of a sonic feed nozzle in order to obtain satisfactory results. The use of a pressure atomizing nozzle system yields increased costs of installation and maintenance which tends to discourage extensive use. U.S. Pat. No. 4,134,729 describes the use of sodium aluminate and n-aminoethyl ethanolamine as a cold end additive and requires the use of liquid atomizer nozzles and a pressure feed system.
U.S. Pat. No. 4,100,258 discloses a method of removing sulfur trioxide by injecting the flue gas with liquid sodium aluminate.